24 



INFRA-RED ABSORPTION SPECTRA. 



Fig. 4a. 



=^ 



two plates of rock salt and permitted to dry. The substances investi- 

 gated did not attack the glue. After filling the cell the top was covered 

 with tinfoil, over which was melted a layer of beeswax. 



A second form of cell, used for liquids boiling above i io°, is shown 

 in fig. 4A. The plates were about 4 by 2.5 by 0.4 cm., split from the 

 natural crystal. Between the common cleavage plane of the two plates 



was placed a washer, iv, of tinfoil, o.oi 

 mm. in thickness, while around the 

 edge was placed a strip of pure tin, t, 

 0.1 mm. in thickness, to prevent evap- 

 oration. This form of cell is much 

 better than that used in previous in- 

 vestigations, in that it can be thor- 

 oughly cleaned, while the washer is 

 discarded for each compound. An- 

 other advantage in using this form of 

 cell is that the tinfoil assumes any 

 small irregularities in the surface of 

 the plates, so that one knows the actual 

 thickness of the film more accurately 

 than in previous forms of rock-salt 

 cells. The cell was filled by placing 

 the washer upon one plate, placing 

 several drops of the liquid upon it, 

 then covering it with the other plate. 

 The tin edge was then put on the out- 

 side and pressed close to the plates. 

 The cell, c, was then mounted in 

 a constant position upon its holder 

 (fig. 4b), which consisted of a heavy 

 block of wood with a narrow opening 

 cut through it. Below the cell was a 

 clear piece of rock salt, r, which was 

 used to eliminate the absorption of the 

 cell, thus enabling one to obtain the 

 transmission through the liquid di- 

 rectly. Furthermore, by using this 

 arrangement no radiation except that which passed through the cell, or 

 through the rock-salt plate, could enter the spectrometer. In fig. 4A 

 is shown the absorption cell, consisting of the rock-salt plates, r, the 

 tinfoil w^asher, zv, and the tin shield, t, while fig. 4B shows the cell, c, 

 mounted upon its holder, with the clear piece of rock salt, r, below it. 



Fig. 4b. 



